Plasma treatment of polymer powders

ABSTRACT

Surfaces of fine polystyrene (PS) and polymethyl methacrylate (PMMA) powders were modified by exposure to the downstream products of a nitrogen or oxygen microwave plasma. The effects of nitrogen and indium incorporation in the powder surface were studied with emphasis on variations in the triboelectric properties of the powder. X-ray photoelectron spectroscopy (XPS) was utilized to determine the changes in surface elemental composition. After nitrogen plasma treatment, the C 1s peak profiles suggested the formation of amines in the case of PS, and the formation of imines and amides in the case of PMMA. Oxygen plasma treatment suggested the formation of hydroxyl and carbonyl groups on the surfaces of both PS and PMMA. After treatment with a nitrogen or oxygen plasma, the charge-to-mass ratio (Q/M) of PS and PMMA powders in contact with carrier particles was measured using the cage blowoff method. The surface charge density (Q/A) was calculated from Q/M. The Q/A of nitrogen plasma-treated PS powder was seen to shift towards positive charge with small increases in the nitrogen concentration. The Q/A of oxygen plasma treated PS powder initially shifted toward negative charge, but changed towards positive charge with higher oxygen concentrations. Plasma-treated PMMA powder showed a different behaviour and the variation of Q/A on PMMA was much less than that of PS. Results suggest that triboelectrification of the polymer powder may be related to changes in the electrical surface states, and that nitrogen may act as a group V dopant within the PS surface.

BACKGROUND OF THE INVENTION

This invention relates to the charging of materials bytriboelectrification, especially the fine polymer powders used as tonersin electrophotographic systems.

The charging of materials by triboelectrification has been applied to anumber of industrial products for some time. Since the invention of theelectrophotographic technique used for copiers and non-impact printersby Carlson in 1938 (see U.S. Pat. No. 2,297,691 (Carlson, 1942)), thefield has developed into a large commercial market. Recently,electrophotography has required higher resolution images than wasnecessary previously, for application in colour image systems (see E.Czech, W. Ostertag, SPIE 1253, Hard Copy and Printing Products, 64,(1990)). For this purpose it is very important to control accurately theelectrical charge of the fine polymer powders used as toners inelectrophotographic systems (see S. Kume, The Institute ofElectrostatics of Japan 10(5), 306 (1986)).

Toners are fine polymer particles typically about 10 μm in diametermixed with various additives and usually include a coloured dye. In atwo-component development system, the toner particles are charged bymaking contact with larger metal beads known as carriers [see L. B.Schein, "Electrophotography and Development Physics", ISBN 3-see540-18902-5, Springer Verlag (1988)]. The toner is transferred to thephotoreceptor due to an attractive electric field to form a real image(development). For a high quality image, it is important to control thecharge-to-mass ratio (Q/M) of the toner within predetermined limits. TheQ/M varies with changes in environmental conditions and surfaceproperties of the toner (see N. Matsui, K. Oka and Y. Inaba, J.Electrophotographics 30(3), 282 (1991)). Much work has been done toinvestigate the triboelectrification of fine polymer powders (see J.Henniker, Nature 196, 474 (1962); C. B. Duke and T. J. Fabish, J. Appl.Phys. 49, 315 (1978); and L. B. Schein and M. Latta, J. Appl. Phys. 69(10), 6817 (1991)), but the mechanisms are still not fully understood.Some investigations suggest that the nature of the chemical species onthe surface is the most important aspect for controlling thetriboelectric charge of the particle (see I. Shinohara, F. Yamamoto, H.Anzai, and S. Endo, J. Electrost. 2, 99 (1976), and H. W. Gibson,Polymer 25, 3 (1984)).

SUMMARY OF THE INVENTION

There is a need to improve the understanding of the relationship betweenthe surface elemental composition and the mechanism of surfaceelectrification on polymer powders, and the present invention hasimproved that understanding. Nitrogen or indium have been incorporatedinto the surface structure of polymer powders using a downstreammicrowave plasma reactor. After this incorporation each of the elementswas found to increase and stabilize the Q/M. A model to explain thechanges in the electrification properties is suggested.

Moreover, there is a need to improve the triboelectric properties ofpolymer powder, and the invention provides a method for doing so,comprising the step of positioning the polymer powder in the afterglowregion of a gas plasma having a main region and an afterglow region,where the gas plasma is in a low-pressure stream of a gas selected fromthe group consisting of oxygen, nitrogen, and gases containing oxygen ornitrogen, whereby low concentrations of oxygen or nitrogen as the casemay be are incorporated into the surface of the powder.

Preferably, the polymer powder is of any conjugated polymer, or apolymer bearing aromatic constituents.

Preferably, the gas is nitrogen, and the polymer is polystyrene orco-polymers of polystyrene.

Alternatively, the method comprises the step of positioning the polymerpowder in the afterglow region of a gas plasma having a main region andan afterglow region, where the gas plasma is in a low-pressure stream ofa gas selected from the group consisting of oxygen, nitrogen, and gasescontaining oxygen or nitrogen, and where indium as a metal foil issuspended in the gas stream adjacent the gas plasma, thereby generatingindium vapour, whereby low concentrations of indium are incorporatedinto the surface of the powder.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic of the plasma reactor for powder treatment;

FIGS. 2(a) and (b) are graphs of the relationship between plasmatreatment time and nitrogen, oxygen concentrations on PS powder;

FIGS. 3(a) and (b) are graphs of the relationship between plasmatreatment time and nitrogen, oxygen concentrations on PMMA powder;

FIGS. 4(a) and (b) are graphs of the relationship between mass of a) PSand b) PMMA powders and the concentration of nitrogen and oxygenobserved;

FIGS. 5 (a) and (b) are graphs of the C 1s spectra of a) nitrogen plasmatreated PS and b) oxygen plasma treated PS powder;

FIGS. 6(a) and (b) are graphs of the C 1s spectra of a) nitrogen plasmatreated PS and b) oxygen plasma treated PMMA powder;

FIGS. 7(a) and (b) are graphs of the relationship between Q/A versus N/Cand O/C for PS powder electrified with a) negative carrier and b)positive carrier;

FIGS. 8(a) and (b) are graphs of the relationship between Q/A versus N/Cand O/C for PMMA powder electrified with a) negative and b) positivecarrier;

FIG. 9 is a graph of the change in Q/A on nitrogen plasma treated PSpowder as a function of time;

FIG. 10 is a graph of the change in N/C on nitrogen plasma treated PSpowder as a function of time;

FIGS. 11(a) and (b) are comparisons of the photoelectron yields ofnitrogen-treated and untreated polystyrene;

FIG. 12 is a graph of changes in Q/A for PS bombarded with In duringIn/Ar plasma treatment;

FIGS. 13(a) and (b) are energy band diagrams for untreated a) PS and b)PMMA; and

FIGS. 14(a) and (b) are energy band diagrams of a) nitrogen plasma andb) oxygen plasma treated PS.

DETAILED DESCRIPTION OF THE INVENTION

1. EXPERIMENTS

Polystyrene (PS) and polymethyl methacrylate (PMMA) powders in the formof small spherical beads were obtained from Sekisuikaseihin Co., (Tokyo,Japan). The XPS analysis was performed using an SSX-100 X-rayphotoelectron spectrometer which utilizes monochromatized Al Kα X-raysfor excitation of the sample. For analysis, the polymer sample was fixedon indium foil (5×5 mm², 0.5 mm thickness) in a sample holder. For theelemental broad scan analysis, the X-ray spot size was set to 600 μm.The elemental broad scan was measured at the three different spots toensure even and overall treatment of the powder. For high resolutionanalysis of the C 1s peak, the X-ray spot size was reduced to 150 μm.The charging of the polymer surface during X-ray exposure was controlledusing the flood gun/screen technique (see C. B. Bryson, Surface Sci.189/190, 50 (1987)). Binding energies have been corrected for the shiftsobserved due to sample charging and spectra are referenced to the C 1shydrocarbon component which was assigned the value of 284.8 eV (see ASTMStandard, E1015, Vol. 03.06 (1984)).

Atomic percentages of oxygen and nitrogen were calculated using Scofieldcross-sections correlated for differences in inelastic mean free pathdue to electron kinetic energy. The inelastic mean free path for C 1selectrons in PS was taken to be 2.9 nm (see R. F. Roberts, D. L. Allara,C. A. Pryde, D. N. E. Buchanan, and N. D. Hobbins, Surface and Interfaceanalysis 2(1), 5 (1980)). The atomic percentages of nitrogen and oxygenwere converted into relative quantities as N/C (nitrogen/carbon atomratio) or O/C (oxygen/carbon atom ratio).

XPS showed the original PS powder (mass mean diameters of 8, 15, and 20μm) to contain some oxygen O/C=0.03±0.01 on the surface. This oxygen isthought to be the result of oxidation during processing. PMMA powder(mass mean diameters of 8, 12, and 20 μm) showed oxygen O/C=0.35±0.01 onthe surface. Since the properties of the powder surfaces change withatmospheric conditions (see Y. Nurata, Hyomen 23 (9), 528 (1985); K. P.Homewood, J. Electrostat. 10, 299 (1981)), it is important to controlthese conditions.

Surface treatment of the powders was carried out in a vortex reactorlocated downstream from a microwave plasma discharge. The vortex reactorused during this work (FIG. 1) consisted of a 100 ml pyrex flask 1 withupper and side necks 2 and 3 respectively. The powder samples 4 wereplaced in the bottom in the flask along with a pyrex covered magneticstirrer 5. The amount of powder sample used for each experiment was0.3-1.2 g. The reactor was connected to the plasma by a 1.5 cm diameterquartz tube 6 fitted into the upper neck 2 of the flask. The upper partof this tube was surrounded by an Evenson microwave cavity 7 connectedto a 120 W, 2.45 GHz microwave generator 8. The side neck 3 of the flaskwas connected to a high volume one stage rotary pump 9 with a pumpingspeed of ≈1.1×10⁶ sccm (standard cubic centimeters per minute). A finecloth filter 10 was placed between the reactor and pump to prevent lossof powder to the pump.

Pure nitrogen gas (99.99%) and pure oxygen gas (99.99%) were used asplasma source gases, introduced via a gas inlet 11. After the reactorwas pumped to an initial base pressure of 5×10⁻² torr, the gas flow wastypically set at a low flow rate (40 sccm) or at a high flow rate (O₂ ;1600 sccm, N₂ ; 2000 sccm) for the experiments. The gas flow rates werecontrolled by a mass flow controller (not shown). The pressures in thereactor were 1.0 torr (at 40 sccm), 3.5 torr (at 1600 sccm), and 5.0torr (at 2000 sccm). The net microwave power for the experiments was setat 40 W. Treatment times were controlled and varied between 5 min and 30min.

In the above described configuration the sample is located ≈16 cmdownstream from the plasma, and is therefore exposed only to the longerlived species in the plasma afterglow region 12. Downstream plasmatreatment of polymer surfaces has previously been studied [see R.Foerch, J. Izawa, and N. S. McIntyre, J. Polymer Sci.: Appl. PolymerSymposium 46, 415 (1990)] and it has been shown that the efficiency ofthe treatment is dependent on the gas flow rate, the microwave powerapplied and the distance of the sample from the plasma (see R. Foerch,N. S. McIntyre, R. N. S. Sodhi, and D. H. Hunter, J. Appl. Polymer Sci.40, 1903 (1990), and R. Foerch, N. S. McIntyre, and D. H. Hunter, J.Polymer Sci., Part A, Polymer Chemistry 28, 193 (1990)).

In downstream plasma treatment, the reactive species of a gas plasma arereacted with a sample positioned a distance beyond the main plasmaregion, i.e. downstream from the main plasma region in terms of thedirection of gas flow. The actual distance could vary from installationto installation. This is known as downstream or remote plasmamodification, the main application of which to date has been in theelectronics and semiconductor industry for the purpose of deposition ofdielectric coatings on semi conductor devices or the cleaning ofpolymers (see Bachman U.S. Pat. No. 4,946,549).

The downstream or remote plasma treatment in the present inventionprovides a much less destructive method for polymer treatment incomparison to direct plasma modification and other more commonly usedmethods such as corona discharge, ozone and flame treatment. Inaddition, the remote plasma treatment enables greater control over thereactive species interacting with the polymer, such that only the longerlived species, i.e. N atoms, reach the sample, rather than a whole rangeof electrons, ions and other excited species.

The cage blowoff method is a simple and reproducible method formeasuring the charge-to-mass ratio (Q/M) of polymer powders in contactwith carrier particles (see L. B. Schein and J. Cranch, J. Appl. Phys46, 5140 (1975). The carrier particles were made of polymer-coatedferrimagnetic particles, with a diameter of 120 μm. The plasma treatedsamples were mixed with the carriers in a 30 ml glass bottle. The ratioof the quantity of sample to carrier was changed with the diameter ofthe sample powders to ensure the same initial ratio of sample surfacearea to carrier surface area (0.5:1) (see N. Hoshi and M. Anzai, J.Electrophotographics 25 (4), 269 (1986). Thus, for a typical powderdiameter of 8 μm, the mixing ratio of sample to carriers was 2.0 wt %,while for 20 μm the ratio was 4.5 wt %. A mixing time of 30 minutes at120 rpm was utilized to electrify the samples sufficiently.

After the mixing, the sample/carrier mixture was transferred to adouble-walled aluminum Faraday cage (the "blowoff cage") with 44 μmmetal mesh covering both ends of the inner container. The smaller powderparticles were blown through the screen using a strong jet of air. Thecharge remaining on the carrier beads was measured using a KeithleyModel 602 electrometer. This charge measured is equal and opposite tothe charge on the powder. The change in mass before and after theblowoff was measured using a balance with an accuracy of ±0.1 mg. Thisdata allowed Q/M to be calculated. Q/M measurements were repeated threetimes on different samples from the same batch of materials within 30min. Two types of carriers were used to electrify the samples eitherwith a negative charge or a positive charge. The difference between thetwo types of carriers lies in the surface polymer compositions of eachof the surfaces.

The charge density Q/A was calculated using the following equation tocompare the samples of different diameters. ##EQU1## where:

Q/A is the charge density [μC/m² ],

Q/M is the-charge-to-mass ratio [μC/g],

d₀ is the density of polymer [g/m³ ],

r₀ is the radius of polymer [m].

The surface elemental composition and the electrical properties of thestarting materials were continuously verified by XPS and measurement ofQ/M.

The results of these experiments are discussed below.

2. RESULTS

a) Plasma treatment of powders

FIG. 2(a) shows the relationship between plasma exposure time and N/C onPS powder for nitrogen gas flow rates of 40 and 2000 sccm. N/C increasedwith treatment time, reaching a maximum of 12% after 20 min at 2000 sccmand 5% after 30 min at 40 sccm. FIG. 2(b) shows the relationship betweenthe treatment time and O/C for PS powder using oxygen gas flow rates of40 and 1600 sccm (the highest stable flow rate for oxygen gas). At 1600sccm the O/C was seen to change from 0.03 to 0.34 within 10 minutes.Decreasing the gas flow rate to 40 sccm showed a similar reaction rateto that at 1600 sccm with a maximum O/C of 0.28 within 10 minutes. Itthus appears that the oxygen plasma treatment is very efficient for PSpowder.

FIG. 3(a) shows the rate of reaction of a nitrogen plasma on PMMApowder. At 2000 sccm the maximum N/C was 0.15 after 10 minutes. UnlikePS, the rate of reaction was seen to change significantly with changesin flow rate. FIG. 3(b) shows that with oxygen plasma treatment, the O/Cof PMMA changed from 0.35 to 0.56 within 10 minutes. Decreasing the flowrate to 40 sccm showed an increase in O/C from 0.35 to 0.45 after 30minutes of exposure. For ease of comparison ΔO/C will be used for PMMA,where ΔO/C indicated O/C of treated PMMA minus O/C of untreated PMMA(O/C=0.35).

The mass of sample used during plasma treatment was changed toinvestigate the effects of the sample quantity in the reactor. FIGS.4(a)(b) shows the relationship between mass of PS(a) and PMMA(b) powderand nitrogen or oxygen concentration. The treatment time was 10 min,using a high gas flow rate. When increasing the mass of samples in thereactor, N/C and O/C decreased for both polymers. These results showthat less quantity of sample is more effective for achieving higherconcentrations of nitrogen and oxygen.

FIGS. 5(a)(b) shows the high resolution C1s peaks of PS powder withnitrogen and oxygen plasma treatment. The C1s peak for untreated PSpowder was resolved into two components. The component of greatestintensity at a binding energy of 284.8 eV represents the hydrocarboncomponent. The feature shifted by 6.7 eV represents the π→π* shake upsatellite characteristic for aromatic or conjugated species (see D. T.Clark and A. Dilks, J. Polym. Sci., Polym. Chem. Ed. 14,533 (1976)).

After exposure to a remote nitrogen plasma (N/C=0.04, O/C=0.06), thepeak shape was seen to alter with the appearance of new functionalgroups (FIG. 5(a)). Since the binding energy shifts for these peaks aresmall (<2 eV), it is believed that the nitrogen adds to the polymer asamine functional groups (C--NH₂, C--NHR, C--NR₂) and imines (C═N).Oxygen appears to add as hydroxyl or ether groups (shift of 1.5 eV).With longer exposure time to a nitrogen plasma (N/C=0.12, O/C=0.07), theintensity of these peaks was seen to increase and a low intensity peakat a binding energy shift of 3.6 eV was observed suggesting theformation of C═O and RCO--NHR groups. A decrease in the π→π* shake upsatellite intensity was seen with remote nitrogen plasma treatmentsuggesting some disruption of the PS conjugated structure.

With exposure to a remote oxygen plasma (FIG. 5(b)) the C 1s peak shapeof PS was seen to change and suggested the formation of hydroxyl groups(shift of 1.5 eV) and carbonyl groups (shift of 3.0 eV) at an O/C=0.10.With higher oxygen concentration (O/C=0.32), the spectrum was seen tochange further with the appearance of additional peak componentssuggesting the formation of carboxyl groups (shift of 4.0 -4.5 eV). Itwas also noted that the π→π* shake up satellite disappeared with longerexposure to the plasma.

The C 1s peak for PMMA (FIGS. 6(a)(b) was also seen to change withplasma treatment. The original material showed four components withinthe peak envelope. These can be associated with the hydrocarboncomponent; C--CO₂ at a binding energy shift of 0.8 eV, C--O at a shiftof 1.5 eV, and the ester carbon RO--C═O at 3.9 eV. With nitrogen plasmatreatment (N/C=0.07, ΔO/C=0.07), small changes are observed the C1sspectrum with an increase in the peaks at 3.9 eV and 0.8 eV and theappearance of a small feature at 3.0 eV (FIG. 6(a)). With furthernitrogen plasma treatment (N/C=0.16, O/C=0.05), an increase in theintensity of all high binding energy peaks was observed. Since O/C didnot change, it can be assumed that this represents the formation offurther nitrogen functional groups such as amines (0.8 eV), amides (3.0eV), and urea type functional groups (4.0-4.5 eV) on the surface [see D.T. Clark and A. Harrison, J. Polym. Sci., Polym. Chem. Ed. 19, 1945(1981)].

With oxygen plasma treatment of PMMA, similar changes in the C 1senvelope were observed (FIG. 6(b)). After a change in O/C of 0.10, theoriginal peak shape changed significantly, with an intensity increase ofthe C--O (shift of 1.5 eV) and RO--C═O (shift of 3.9 eV) components.With longer exposure times (ΔO/C=0.22) an extra feature appeared at 3.0eV indicating the formation of C═O groups on the surface. This appearedto be accompanied by an intensity decrease of the C--O and RO--C═Ogroups, which may suggest disruption of the PMMA polymer structure.

b) Electrical characteristics

Polystyrene:

FIG. 7(a) shows the relationship between the relative concentration ofnitrogen (N/C) or oxygen (O/C) and the charge density (Q/A) on the PSpowder electrified using a negative carrier. Powder samples of diameter8 μm (Δ), 15 μm (□), and 20 μm (◯) were used. Since all measured pointsfit on the same curve it is evident that the charge density isindependent of the diameter of the powder. Untreated PS powder showed aQ/A of about -120 μC/m². Even after very brief nitrogen plasma treatmenta significant change towards positive charge was observed. Q/A reached amaximum (0 μC/m²) when XPS indicated N/C to be 0.08-0.09. No furtherchange in Q/A was observed with higher nitrogen content. This effect wasfound to be even more rapid when a positive carrier was used (FIG.7(b)). Untreated PS powder did not accumulate charge when in contactwith the positive carrier (Q/A=0 μC/m²). However, after nitrogen plasmatreatment to N/C=0.02, Q/A shifted toward a maximum positive charge of+110 μC/m². The difference in Q/A before and after nitrogen plasmatreatment, ΔQ/A, was similar for both negative and positive carriers,but the effect of low nitrogen surface concentrations was much morepronounced with a positive carrier.

In contrast to the changes caused by the nitrogen plasma treatments,oxygen plasma treatment of polystyrene resulted in markedly differentcharging behaviour. In FIG. 1 electrification of oxygen plasma treatedPS with a negative carrier caused its Q/A to decrease under conditionswhere the surface oxygen content is relatively low. However, with higheroxygen concentrations a minimum in Q/A is reached. Then, at still higheroxygen concentrations (O/C=0.1) the Q/A increases. Electrification ofoxygen plasma treated PS particles with a positive carrier was found torespond to oxygen surface concentrations in a manner qualitativelysimilar to electrification with a negative carrier.

Polymethylmethacrylate:

Similar experiments using PMMA powder showed a very different behaviourthan that of PS. FIGS. 8(a)(b) show the relationships between the N/C orΔO/C and the Q/A of PMMA, electrified by negative (a) and positive (b)carriers. In this graph again several powder diameters were plottedtogether (diameter 8, 12, 20 μm). The value of Q/M of untreated PMMA wasfound to be about +85 μC/m² using a positive carrier. Q/A was found toincrease only slightly (+100 μC/m²) for low nitrogen concentration(N/C<0.07) and then actually decreased with higher values of N/C. Q/Mdecreased with an increase in ΔO/C. A very rapid change in Q/A wasobserved near ΔO/C=0.10 for both carriers suggesting significant changesin the surface properties at that point.

The data observed for PMMA in fact showed no resemblance to that of PSsuggesting very different charging mechanisms for the two polymers.

c) Surface aging

Q/A on the plasma treated PS and PMMA was measured for several weeksafter the plasma treatment in order to investigate the effect of agingon the ability of the particle to accumulate charge. FIG. 9 shows thechange in Q/A for PS powder using negative carrier. For high nitrogenconcentration (N/C=0.12) Q/A did not appear to change over a period of120 days. For low nitrogen concentration PS (N/C=0.03) Q/A actuallyincreased to a more positive value (20%) over 130 days. The Q/A ofoxygen plasma treatment PS was seen to decrease to a more negative value(about 2-5%) over 140 days. The concentration of nitrogen (N/C) on thePS and PMMA was also measured for several weeks after the plasmatreatment to investigate the aging phenomemon. FIG. 10 shows the changein the N/C ratio on PS powder as a function of time. The nitrogenconcentration was seen to remain within experimental error over the 120days for samples with low surface nitrogen concentrations. These resultssuggest that while the concentration of oxygen and nitrogen changesslightly with time, the surfaces retain their electrical properties.

d) Photoemission Studies

The photoelectron yields of nitrogen-treated and untreated polystyrenehave been measured using a dedicated instrument. In FIGS. 11(a) and11(b), the photoelectron yields are compared. Both samples have workfunctions close to 5.05 eV, but the electron yields above this energy isgreater than ten times higher for the nitrogen-treated surface.

e) Effect of Indium

Other elements have also been found to affect the charging properties ofpolystyrene when introduced in low concentrations into the surface. Forexample, indium added to the surface in quantities of 1 atomic % causedthe Q/A to increase to similar values as was found for nitrogen.

Indium as a metal foil was suspended in the gas flow just below themicrowave cavity. A gas flow of 1000 sccm was used to transfer indiumvapor to the surface of polystyrene particles in the normal place in thereactor shown in FIG. 1. XPS showed that a small concentration of indiumwas present on the surface as In⁺³. The presence on the surface of lowconcentrations of indium caused a major increase in the electricalcharge retained, as shown in FIG. 12.

3. DISCUSSION

These results have shown that, by using a downstream nitrogen plasma, itis possible to make major and long-lasting changes to the triboelectricproperties of PS powder. Q/A measurements of nitrogen plasma-treated PShave shown dramatic changes in charging properties with very lowincorporation of nitrogen into PS. In order to understand thisphenomenon, Q/A data must be carefully compared to the available XPSdata. XPS analysis has suggested that during nitrogen plasma treatmentthe major species formed are amines, even at higher N/C (FIG. 5(a)). Q/Ameasurements have shown a very rapid shift towards positive chargeduring the initial stages of nitrogen incorporation. The charge reachesa plateau well before the surface amine concentration has reached itsmaximum. The effect is particularly marked when using a positive carrierfor electrification.

This has led the inventors to believe that the change in Q/A is notentirely controlled by surface chemistry and interfacial adhesionforces, but may also be affected by changes in the surface electronicproperties. Polystyrene, on the basis of calculations of band gap [seeC. B. Duke and T. J. Fabish, J. Appl. Phys. 49,315 (1979)], is believedto have few surface states near its Fermi level (FIGS. 13 (a)(b)). Asmall concentration of nitrogen (N/C<0.03), acting as a donor, couldcreate additional unoccupied surface states in this region (FIG. 14(a))which may stabilize charges. The effect of the nitrogen on thephotoemission yield provides evidence for an alteration of electronicband structure. The differences in the effectiveness of positive andnegative carriers may, however, be governed by surface chemistry. For apositive carrier even low concentrations of nitrogen impart the maximumeffect on the Q/A of polystyrene particles. The less dramatic effectachieved with the negative particle may be due to a difficulty inachieving good contact between carrier and toner until the toner surfacehas sufficient hydrophylicity brought about by extensive nitrogentreatment.

XPS results have shown that the main structure of PS has remained, evenat high nitrogen concentration, but with some loss of aromaticcharacter. Loss of the conjugated structure could alter significantlythe charging properties by altering the density of occupied andunoccupied states.

Oxygen plasma treatment results for PS suggest that rather differentmechanisms are involved. Q/A increased towards a small negative chargeat low O/C and slowly decreased towards positive charge at O/C>0.05 or0.10 for negative and positive carriers respectively. The oxygen addedmay create both occupied and unoccupied surface states which couldchange the Fermi level (FIG. 14(b)).

In contrast to PS, PMMA has many surface states in the band gap. The twoband gaps are compared in FIG. 13. The band gap states are thereforeless sensitive to the addition of potential donor atoms such asnitrogen. The effect of nitrogen addition on Q/A for PMMA may only bethe result of changes in surface charging. The change of Q/A on theoxygen plasma-treated PMMA is also much less than that for PS. A rapidchange near O/C=0.10 of Q/A on oxygen plasma-treatment PMMA may becaused by the disruption of the PMMA polymer structure (FIG. 6(b)).

In conclusion, the surfaces of PS and PMMA powders were modified bytreating them with downstream nitrogen and oxygen plasmas. Q/A ofnitrogen plasma-treated PS powder has shown a very rapid change towardspositive charge with small increases in N/C. It is believed thatnitrogen atoms could act as a donor and increase the unoccupied surfacestates in the surface of PS. The variation of Q/A of PMMA has been muchless than that of PS, perhaps because of the larger number of surfacestates in the band gap.

We claim:
 1. A method of improving the triboelectric properties ofpolymer powder, comprising the step of positioning said polymer powderin the afterglow region of a gas plasma having a main region and anafterglow region, where said gas plasma is in a low-pressure stream of agas selected from the group consisting of oxygen, nitrogen, and gasescontaining oxygen or nitrogen, whereby low concentrations of oxygen ornitrogen as the case may be are incorporated into the surface of thepowder.
 2. A method as recited in claim 1, where said polymer powder isof any conjugated polymer.
 3. A method as recited in claim 1, where saidpolymer powder is of any polymer bearing aromatic constituents.
 4. Amethod as recited in claim 1, in which said gas is nitrogen, and inwhich said polymer is selected from the group consisting of polystyreneand co-polymers of polystyrene.
 5. A method of improving thetriboelectric properties of polymer powder, comprising the step ofpositioning said polymer powder in the afterglow region of a gas plasmahaving a main region and an afterglow region, where said gas plasma isin a low-pressure stream of a gas selected from the group consisting ofoxygen, nitrogen, and gases containing oxygen or nitrogen, and whereindium as a metal foil is suspended in said gas stream adjacent said gasplasma, thereby generating indium vapour, whereby low concentrations ofindium are incorporated into the surface of the powder.